Stroke Risk After Cardiac Surgery
An estimated 330,000 surgical procedures were performed using cardiopulmonary bypass (CPB) in 1994 in the United States (Mills, 1995). With the increasing age and incidence of concomitant disease, it is increasingly recognized that emboli from instrumentation of an atherosclerotic aorta is an important source of stroke and central nervous system (CNS) morbidity (Murkin et al., 1995; Blauth et al., 1992; and Tuman et al., 1992). There is a direct correlation between age, peripheral vascular disease, and insulin dependent diabetes mellitus (IDDM) and severe atherosclerosis of the ascending aorta and atheroemboli production (Blauth et al., 1992). In a large postmortem study of 221 patients dying after cardiac surgery, atheroemboli were present in the brains in 37% of patients with severe disease of the ascending aorta but only 22% of the patients without severe disease (Blauth et al., 1992). 95% of patients who had evidence of atheroemboli postmortem (and would have manifested all the signs of a stroke had they lived), had severe atherosclerosis of the ascending aorta (Sylviris et al., 1997). In a study of 2000 CAB patients, Tuman et al, (1992), reported an overall postoperative stroke rate of 2.8%, but in patients 65 to 74 it was 3.6%, and in those over age 75 the stroke rate was 8.9%. Currently 30 to 40% of the population we operate upon for coronary bypass surgery is in this age range. Patients with a postoperative neurologic event had a ninefold increase in mortality (35.7% versus 4.0%).
Current Detection of Aortic Plaque
In fewer than 50% of patients can the presence of aortic arch atheromatous disease be predicted preoperatively using chest X-ray (CXR), or aortogram (Hosoda et al., 1991). Furthermore, 50-80% of significant atherosclerotic lesions in the ascending aorta are missed by intra-operative palpation by the surgeon (Hosoda et al., 1991; Davila-Roman et al., 1994; Barzilai et al., 1989; Marschall et al., 1989; and Katz et al., 1992). Katz et al., (1992), found that in a prospective study involving 130 patients, 19 (83%) of 23 patients with severe disease by TEE were graded normal or mild by palpation. While calcific aorta can be assessed reasonably well, "Cheesy" atherosclerosis is extremely difficult to detect by palpation (Landymore and Kinley, 1983). Manual palpation of the aorta by the surgeon to assess for optimal cannulation sites is currently the standard of care in most cardiac surgical centers in North America. Identifying severe aortic disease has important clinical implications because surgical technique, including aortic cannulation to connect to the heart-lung machine (cardiopulmonary bypass, CPB machine) and anastomosis of proximal coronary grafts, and other such interventions may be altered or relocated to avoid areas of atherosclerotic plaque and should reasonably result in a decrease in stroke rate and in mortality associated with patients undergoing cardiac surgery (Hosoda et al., 1991; Davila-Roman et al., 1994; Barzilai et al., 1989; Marschall et al., 1989; Katz et al., 1992; and Wareing et al., 1992)
Intraoperative Aortic Scanning
Rather than manual palpation, intra-operative ultrasound studies of the aorta using transesophageal echocardiography (TEE) of the aorta has been recommended as a routine in order to detect aortic atherosclerosis and guide surgical cannulation etc (Hosoda et al., 1991). However, 1) this is an expensive instrument (average $125,000-$500,000 capital cost), 2) requiring significant expertise and an independent dedicated operator (presence of a dedicated technician or specially trained physician) for its intraoperative usage, and 3) its ability to detect all aortic arch lesions has been questioned since the air-tissue interface resulting from the lung and trachea prevents the identification of lesions in the upper ascending aorta and the aortic arch, where cannulation is done (Seward et al., 1990; Konstadt et al., 1994; Sylviris et al., 1992; and Kanchuger et al., 1994).
Alternatively, employment of a hand-held epiaortic B-mode scanning probe has been shown to be more efficacious than TEE and similarly alters the site of aortic cannulation and instrumentation in 20-24% of CPB cases (Barzilai et al., 1989; Ohteki et al., 1990; and Davila-Roman et al., 1991). Epiaortic B-mode scanning has been shown to be accurate to assess severity and location of atherosclerosis of the ascending aorta and allow modification of the standard technique for cannulation by choosing a safer site (Davila-Roman et al., 1994 and Wareing et al., 1992). Epiaortic scanning has been found to more reliable in identifying plaque in the distal ascending aorta where TEE is less helpful. Katz and colleagues (1992) showed that all 5 patients in whom severe distal ascending plaque was found by direct epiaortic probe were missed by biplanar TEE. The use of this instrument would obviate the need for manual palpation of the aorta, in itself a cause of embolization (Karalis et al., 1992).
Currently the standard of care continues to be visual inspection and palpation of the aorta by the surgeon, despite the fact that it has been shown to identify atheromatous disease in only 25-50% of patients and even then underestimates atherosclerotic severity compared with ultrasound scanning (Seward et al., 1990; Konstadt et al., 1994; Sylviris et al., 1992; and Kanchuger et al., 1994).
A further problem relates to the inability to reliably remove air and particulate debris from within the heart after open-heart surgery (valvular surgery, septal defect repairs, congenital heart surgery) because of an inability of the surgeon to view the interior of the heart chambers. Current techniques employ blind needle aspiration of the heart chambers which is unreliable for effective removal of such foreign matter. Use of TEE enables visualization of air/debris within the heart chambers but does not assist with localization of the tip of the aspirating needle since it is introduced from a separate site. Thus the needle aspiration is performed `blindly` and is only randomly in contact with air within the heart cavity.
Traditional medical ultrasound devices are not designed to be operated and controlled from within a sterile surgical field. They are designed to be controlled and run by an external operator (technician) with the surgeon acting to position the probe within the surgical field. Accordingly they require ancillary support personnel who must respond to input from the surgeon or other individual manipulating the probe in the operative field in order to obtain the best images of the tissue under study. Further, current medical ultrasound devices are relatively large machines designed for a variety of other imaging applications, generally external to the body. In the restricted space of the operating room, this large size is potentially hazardous as it may block access to the patient or other necessary medical equipment. There are smaller general purpose ultrasound devices, but they have a very small display and are designed to be viewed at close range. They are thus rendered difficult to see when viewed from within the operating field and they also cannot be controlled from within the surgical field.
Because of a multiplicity of uses traditional medical ultrasound devices are complex to operate and require the presence of a trained technician. Additionally, the imaging controls are located on the nonsterile housing of the ultrasound device. Thus, an operator is not able to control the ultrasound scanning device independently from within the sterile surgical field.
A related problem associated with current scanning devices is that scanning probes placed directly on tissue often results in a loss of near field resolution. Use of gel offsets have been employed to enhance near field resolution, however, this has resulted in an inability to identify the location of the tissue being imaged with precision, due to the opaque nature of gel offsets. Furthermore, in this respect, there is currently no ability to precisely orient a scanned image with respect to anatomical features of tissue which is being imaged.